A train passes two persons walking in the same direction as the train. The time it takes to move past the man running at 4km/h is 6sec, whereas the time it takes to cross the other man running at 11km/h is 7sec. What is the speed of the train?
Let the speed of the train be v km/h. Length of the train calculated with the data for the first man = $(v - 4) × 6$. It should equal the length obtained from the data for the second man. So $(v - 4) × 6$ = $(v - 11) × 7$. Please note that we have not converted seconds to hours because that factor will ultimately cancel away. Solving for v we get 53km/h.